A birefringent pattern in which regions having different birefringence are arranged in a patterned manner is used for an optical compensation film of a liquid crystal display device, an authentication image for preventing forgery, an optical element, or the like. The birefringent pattern has been conventionally produced by subjecting a birefringent pattern builder to heating or exposure in a patterned manner.
As the birefringent pattern builder, a polymerizable liquid crystal material containing a photoisomerizable compound is disclosed in JP-T-2006-526165 (“JP-T” means a published Japanese translation of PCT international application). By partially isomerizing this material, and subsequently polymerizing and fixing the material, a birefringent pattern is produced.
United Kingdom Patent No. 2394718A discloses a method of producing a phase difference pattern by curing a polymerizable liquid crystal material based on different conditions depending on the respective regions. JP-A-3-141320 (“JP-A” means unexamined published Japanese patent application) discloses a birefringent pattern produced by a thermoplastic/photodegradable phase difference material.
In the conventionally-known method as described above, the materials are in a monomer form, and they are not in a film form until the birefringent pattern is produced. Thus, it is difficult to handle these materials. Furthermore, the materials once formed in a film-like shape have therein a fixed birefringent pattern. Therefore, it is impossible to use the materials as a material for producing a birefringent pattern.
A seal displaying an image for which the imitation is difficult is helpful, by being adhered on a product, for preventing forgery of the product. However, the seal form may cause a case where the seal is used for a fake product.
As a technique for preventing the seal from being used for a fake product, JP-A-8-95491 discloses a brittle seal containing a partially-formed delamination layer. According to this technique, on a functional layer obtained by laminating a plurality of ceramic materials having different refractive indexes for determining the authenticity, a partially-formed delamination layer is provided at which a part of the functional layer can be peeled. Once the seal is peeled, a display that is visualized based on a difference in the optical property between the part at which the seal is already peeled off and the residual left on the adhering base functions as an indicating sign for preventing the seal from being used for a fake product. However, the laminated structure material having different refractive indexes does not have a latent image that can be clearly determined by a polarizing plate or the like and thus is not optimal for the judgment of the authenticity.
JP-A-9-244519 discloses a technique regarding a seal having a hologram provided on a base material that is used to conduct an authenticity judgment. This seal is structured so that a surface having regions different in adhesive between the base material and a hologram-forming layer is provided by subjecting the base material to a processing (e.g., corona processing). Specifically, due to the difference in the adherence property between the processed part and the not-processed part, the seal is broken when the seal is peeled. However, with the spread of techniques, the production of holograms has been easier, resulting in a current situation where more visual holograms are produced that cannot be distinguished from the authentic ones.
A birefringent pattern through which a latent image is visualized by a polarizing plate is one of techniques suggested to be used as an authentication image for distinguishing the false one from the true one (JP-A-2007-1130). However, no seal has been known that can be used for the authenticity judgment, that has a birefringent pattern having a high resolution and a high heat resistance, and that has a birefringent pattern prevented from being used for a fake product.